Definitely not only small cities. Only dense cities maybe, but I'm not sure that the causality link is in the direction you present: big european cities are dense in part _because_ of public transport. They couldn't be so dense without it, you'd need much more car infrastructure (roads, parking lots) which already decreases density, and people would more readily live further out given they'd already need a car anyway.

It's obviously a virtuous cycle, but in the case of European cities, they are the way they are because they began when walking was the preferred mode of transport ("preferred" is doing a lot of heavy lifting in this sentence, I know). The most luxurious transport option back then was a horse carriage, which is not much faster than walking. Everything had to be close to everything, streets had to be narrow.

Another important feature that makes public transport suitable is that they developed concentrically, having only one city center.

With such city planning in mind, public transport is the only rational way to move around lots of people on narrow streets.

European cities, geographically at least, are a lot smaller than american cities. A dozen or two miles across only for the urban area before you get to the rural parts generally. Berlin has a smaller footprint than a lot of rust belt cities for example. Amsterdam is downright tiny by american standards, maybe even smaller than Toledo, OH. What is more is the road capacity is significantly overbuilt in the US, even in cities like Toledo. A lot of americans have a car based frame of reference where they expect to go arbitrary 30 mile distances in about 30 mins.